The Fall and Rise of the I-35W Mississippi River Bridge – Part 2: Structure | streets.mn

Cross-posted from streets.mn: The Fall and Rise of the I-35W Mississippi River Bridge – Part 2: Structure

The Fall and Rise of the I-35W Mississippi River Bridge – Part 2: Structure

 

Bridges are designed to overcome gravity. They take travelers over a trench, river, or chasm of some kind to reduce the costs of travel. In their absence, travelers would need to descend, and ford a river, take a ferry, or make some other less convenient accommodation. Bridges are networks, sometimes simple, sometimes complex, for transmitting forces from the air to the ground. These networks may be of stone, concrete, wood, steel, or other materials. The network elements are connected in various ways.

The I-35W Bridge was constructed as part of the Interstate Highway System. It was not the first crossing of the Mississippi River in the City of Minneapolis, one can see many other crossings from the photos and maps. Immediately upstream we find the oldest extant crossing, the curved Stone Arch Bridge, dating from the 1883, which originally brought trains of the Great Northern Railway across the Saint Anthony Falls from Old St. Anthony on the east bank of the River to the Mill District on the West Bank, and now acts as a pedestrian crossing. Immediately downstream is the 10th Avenue Bridge, opened in 1929, and still carrying vehicles. The first river crossing in Minneapolis was the 1855 Hennepin Avenue Bridge, a tolled suspension bridge, which lasted at least 20 years.

In principle engineers know (or knew) how to build long lasting structures, that with proper maintenance could last centuries. In fact the Pons Fabricius in Rome was originally constructed in 62 BC, more than 2,000 years ago, and has remained in continuous use. So something went wrong on I-35W for it to last only 40 years, and something has gone wrong in civil engineering practice if we are designing bridges to only last 50 years.

The National Transportation Safety Board, the federal government agency for investigating failures, engaged in an extensive one-year study of the collapse. Inadequately sized gusset plates, sheets of steel that connect truss members, beams, girders, and columns in bridges and other structures, were the proximate cause. While the gusset places were too thin for the design, they were not so thin that the Bridge collapsed earlier. As can be seen from the pictures, the Bridge was undergoing some construction at the time of the collapse, only two of the four lanes were open to traffic, while the others were being resurfaced. It was the combination of the undersized gusset plate with increased weight of the Bridge over time (due to things like pavement resurfacings), and in particular, the loading of construction materials on the Bridge, above the gusset plate that day was the proverbial “straw that broke the camel’s back”. Once one gusset place cracked and could not support the loads, cascading failures led to the collapse (A summary of the NTSB report can be found at “NTSB releases report on I-35W bridge collapse” in Roads\&Bridges, November 17, 2008 (Accessed April 19, 2012)). The Bridge was fracture critical or “non-load-path-redundant”, meaning that once one critical element failed, there was no redundant element to take the load.

Tom Fisher says that fracture-critical design has four characteristics: lack of redundancy, interconnectedness, efficiency, and sensitivity to stress (Fisher, Thomas (2009) Fracture Critical.  Places: Design Observer.(Accessed April 19, 2012)). Beyond that, it has long been known the Bridge was structurally deficient, and it had been investigated for other possible failure modes.

A report by my late colleague Bob Dexter is interesting in that it said
“As a result, Mn/DOT does not need to prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project.” The report was correct as far as it went, since fatigue cracking was not the source of failure. It did not identify the problems with the gusset plates, nor did any inspections after construction. (Robert Dexter, Heather O’Connell, Paul Bergson (2001) Fatigue Evaluation of the Deck Truss of Bridge 9340. Report no. Mn/DOT 2001-10 . The full report NTSB/HAR-08/03 PB2008-916203.)

The US still has about 18,000 fracture critical bridges (America’s Broken Bridges
By Carol Wolf on March 22, 2012). Some 465 have similar designs to the I-35W Bridge. There are about 72,500 structurally deficient bridges according to USDOT, out of about 600,000 bridges (Transportation Statistics Annual Report, U.S. Department of Transportation, Bureau of Transportation Statistics, 2008). Another 80,000 are functionally obsolete, which does not imply a bridge safety problem, but means they are not to standard, for instance with narrow lanes, or are under-capacity for demand.

Bridge failures on the Interstate are not as uncommon as one might think. The list below shows major Interstate bridge failures and their causes.

  • Tampa Bay, FL – May 9, 1980 – I-275 – ship collision
  • Greenwich, CT – June 28, 1983- I-95 – metal corrosion, fatigue
  • Oakland, CA – October 17, 1989- Bay Bridge- earthquake
  • Oakland, CA – October 17, 1989- I-880- earthquake
  • Milwaukee, WI – December 13, 2000- I-794 – weather, traffic?
  • Webbers Falls, OK – May 26, 2002- I-40 – barge collision
  • Bridgeport, CT – March 2003 – I-95 – car-truck fire
  • Oakland, CA – April 29, 2007- MacArthur Maze – truck explosion
  • Minneapolis, MN – August 1, 2007 – I-35W – design, construction

Other bridges have been closed before failure, and repaired or replaced. The Sherman Minton Bridge across the Ohio River was closed in 2011 after cracks were discovered, and repaired. While some causes seem to be acts of nature (earthquakes) or difficult to predict (barge collision, truck explosion), good design will defend against even those failures, at least to a point. The trade-off inherent in all design is the amount of failure to be accepted. Will we accept one Interstate bridge failure in the US every day (no), every month (no), every year (no), every decade (yes), or every century (yes)? Nine failures in 27 years indicates about one every three years is somehow acceptable.

Each higher standard is increasingly expensive. At some point, money spent on reducing fatalities by making ever safer bridges outweighs the same money spent on reducing deaths some other way (e.g. increasing traffic safety or reducing air pollution). For instance, a billion dollars annually spent on reducing expected fatalities from bridge collapses by one person per year is 200 times more than would be spent reducing traffic fatalities (where the “statistical value of life” is on the order of 5 to 6 million dollars per person), and would be a misallocation of resources from a safety perspective.

We can of course potentially add the costs of infrastructure replacement avoided, but we currently spend more per life saved on safety in structures than on safety in traffic. As with aviation crashes, bridge collapses are highly visible and are perceived as more common than they really are.

 

Other Parts in Series: Part 1 – IntroductionPart 2 – StructurePart 3 – CommunicationPart 4 – PoliticsPart 5 – EconomicsPart 6 – TrafficPart 7 – ReplacementPart 8 – Policy Implications

The Fall and Rise of the I-35W Mississippi River Bridge – Part 1: Introduction | streets.mn

Now at streets.mn : The Fall and Rise of the I-35W Mississippi River Bridge – Part 1: Introduction

 

The Fall and Rise of the I-35W Mississippi River Bridge – Part 1: Introduction

C: Starting Date Wednesday August 1 2007, at 6 hours, 5 minutes and 38 seconds pm 

V1: Metro, fifty cars there’s report of some sort of collapse in the construction zone. North of University [Avenue]

V2: 437

V3: 551 I will be 10-8 [available for incidents] down here, she’s on her way doing triple A now

V4: 224

V5: Metro, West Metro’s going to be 10-33 [alarm sounding, emergency] at this time, all cars, fifty cars, we have a bridge collapse, the River Bridge over the Mississippi River Bridge is down

V5: 2500 do you copy?
[siren in background]

V6: 80 metro 60 cars being routed metro

V5: We will need southbound closed and northbound, both sides are down

V5: 2500 do you copy?

V7: In route 169 to 97

V8: 10-4 [ OK, I acknowledge] 1806

C: Ending Date Wednesday August at 6 hours, 6 minutes and 53 seconds pm

Note: C: is the computer time stamp, Vn: are the different voices heard on the recording. The 10 codes have been defined in [brackets]}“10-codes” are widely used by police and emergency personnel in the US to convey information, and were first published in 1940, though there is great debate as to their usefulness compared with plain speech. See Dispatch Magazine online for some discussion.

After spending 10 months on sabbatical in London studying the coevolution of transport and land use, on Tuesday July 31, 2007, my family and I returned home to Minneapolis. We had many things to do to restart a household, among them shopping. Relatives were in town for a conference, and they had rented a minivan. The next day we went to a warehouse club (Costco) to stock up on basic stores, filling the back of our vehicle with typical American middle class goods (paper towels, diapers, etc.). While we traveled to the store on what any online mapping service would suggest is the shortest path, on the return at about 3:00 pm we took West River Parkway instead of the highway to avoid traffic and have a more scenic view. That route runs along the Mississippi River, and passes immediately below the I-35W Bridge. I did not look up at the Bridge from below as we drove under it.

At 6:05 pm, CDT, August 1, 2007, the I-35W Mississippi River Bridge famously collapsed.

By 6:10 pm, I and the world knew about the collapse from watching both local news and CNN. People from around the world contacted me wishing well.

I didn’t know any of the 13 dead or 145 injured at the time, and was as surprised as anyone at the collapse, having driven under and on the Bridge many times. We discovered that it was an eight-lane truss arch bridge that had opened in 1967, and carried about 140,000 vehicles a day. Everyone in the Greater Minneapolis-St. Paul region has their own story, some heroic, most mundane. Everyone, though, remembers it.

The Regional Traffic Management Center (RTMC) in Roseville, Minnesota received news instantly, and had video cameras in the area, which they quickly pointed in the direction where once stood the Bridge. The recording of their audio inputs are transcribed in the opening quote. In the stream of random and mundane information coming into the center, communications were received about the collapse.

The next morning (August 2, 2007 – 8:24 AM) Paul Levy of the local Star Tribune newspaper reported an article with the headline (one which varied across the day and week)
4 dead, 79 injured, 20 missing after dozens of vehicles plummet into river .The article can be found online.

As with any tragedy, information as of August 2 was incomplete. People were missing, some of whom were found alive, others dead. The estimates of injured went up as better counts were made.

In the days following, I received some 17 media contacts asking about the traffic effects. My structural engineering colleagues received many, many more. As researchers, my transportation colleagues and I quickly proposed studies to examine the consequences of the collapse.

Users take infrastructure for granted. From the roots “infra” meaning below or underneath, and “structure” meaning building or assemblage, infrastructure is by its very nature not obvious, and is often hidden in plain sight. Yet its absence is noticed. Americans seldom complain about lack of on-demand electricity (blackouts), natural gas, or water, but often complain about lack of on-demand transportation capacity, which we call congestion. When construction or events close routes, so you cannot get from here to there, the complaints rise. But when infrastructure fails unexpectedly, it engenders shock rather than complaint.

Why did the Bridge collapse? And what does it say about the state of infrastructure in the United States, and for that matter, the developed world?

In a one-time event, blame is a useless exercise. My blaming you will not produce better future outcomes. But in a world with signals and repeated games, blame can lead people to behave better in the future, and the prospect of being blamed for failure may encourage behavior to avoid failure. Too much blame for failure, and insufficient reward for success, will lead to risk-averse outcomes. In some arenas, conventional finance, structural engineering, risk-aversion is probably a good idea. They provide a lattice on which the rest of society depends to accomplish their own work. The gains from innovation are likely small, the losses higher. In other areas, e.g. war, risk-aversion on the part of the weaker army may ensure defeat.

To answer the question on “Why the Bridge collapsed”, one can look to physics, and blame gravity. Blaming gravity, while technically correct in some sense, will not help us going forward.

One can look to structural engineering and blame undersized gusset plates. Or one can look to construction engineering practices and blame overloading. Or one can look to traffic engineering, and blame the need for all those people to get from A to B. Or one can look to politics, and ask why the Bridge which had (different) known problems, had not already been repaired. And so on. The layers of blame are worth exploring.

The collapse of the Bridge illustrates several different kinds of networks in action. The bridge itself was a structural network, a connection of steel and concrete elements designed (but in the end failing to) to transmit force safely from the Bridge deck to the ground. The Bridge was a link in the transportation network, an element of the limited access US Interstate Highway System enabling people to travel by car from point to point without stopping. The news of the collapse of the Bridge was transmitted over communications networks (both electronic and social), it was a quickly transmitted piece of information.

 

Other Parts in Series: Part 1 – IntroductionPart 2 – StructurePart 3 – CommunicationPart 4 – PoliticsPart 5 – EconomicsPart 6 – TrafficPart 7 – ReplacementPart 8 – Policy Implications

 

Light-rail service suspended, bridge, road closed after cable problem

Star Tribune: Light-rail service suspended, bridge, road closed after cable problem : “The failure of a cable support on the Sabo bike and pedestrian bridge has resulted in closure of the bridge, suspension of light-rail service at three stops and the rerouting of vehicle traffic on Hiwatha [sic] Avenue.”

An Economic Comment on the Stillwater Bridge

Jason Scheppers writes in:

 

“Recently, Dr. Whitehead wrote regarding the Stillwater – St. Croix River Crossing. I have seen in your blog several mentions of the bridge and offer you the following economic comments:

I would recommend the following documents for any interested in the details of the current controversy: US House subcommittee hearing, Senate subcommittee Hearing, Record of Decision and US Court ruling vacating the National Park Service’s concurrence with the project.

I support Senators Klobuchar and Franken’s and Rep. Bachman’s right to follow the law which specifically called for override of the Scenic by-way by congress, if Congress deemed appropriate. The tremendously cumbersome process and triple flip-flopping by some federal agencies give significant cause to provide reasonable congressional relief.

But beyond the Congress’s right, the following shows that the economics constructing a new freeway bridge may not be as clear as suggested.

First, the existing bridge while currently rated in not so good condition and with load restrictions is not in any imminent state of collapse. It is also true that under build the new freeway bridge(s) would keep the existing bridge and make it a pedestrian and bicycle facility. The loading requirements for pedestrian facilities exceed those of vehicular loading due to possible densities of pedestrians during special events. The existing historic bridge is not going away.

Second, the I-94 Bridge is only 6 miles away from the current bridge. Attached are the Google directions for a path that goes over the existing bridge as compared to going over I-94, yielding 31 miles in 45 minutes, versus 34 miles in 46 minutes. The new route takes out the trip through Stillwater and straightens out some of the wiggles through Stillwater. I estimate that the time savings for the mean traveler is in the order of 10 minutes and 5 miles compared to the I35 route. Time valued at $15 per hour and cost of $0.40 per mile yields a total savings of $4.50. There actual has been a study done that found the revenue maximizing toll was $3.00. It found that the toll could cover only half the cost of the bridge.

Third, the cost benefit analysis provided in the Supplemental EIS had a very different take. It shows a 6.0 Benefits to Costs ratio. Call me old fashioned but if you can only generate enough toll revenue to pay for half of your project, it is hard to see how the B/C ratio could be greater than one. (There may indeed not be a toll on the new bridge, but the willingness to pay aspect of a toll illuminates the value of the facility.)

Fourth, the travel patterns of the residents of the area are highly dependent on what facilities are in place. Does the Minneapolis Regional model takes into account the natural barrier the river is to growth on the WI side of the St. Croix? The Toll study cited above assumed 2.0% traffic growth, but if you analyze the data in the FHWA vmt trends for Minnesota you will find an annual state wide traffic growth rate of 0.6% from 2004 to 2010. This implying that there is substantial risk to cover 50% of the costs with toll revenues. If this lower traffic growth rate were sustained over the length of the tolling not even 20% of the bridge could be financed.

So enough complaining, here are some things that I think would be reasonable. While the reports all document the danger, it is not the bridge itself where the DOTs are claiming most dangerous conditions. It is on the Minnesota side where the highway goes through the town. It is also the signals through town that impede the volume. The bridge can likely handle 18,000 vehicles a day in each direction without substantial delay. But looking at the route through

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Stillwater, one possible explanation for the accidents is the on street parallel parking on this high volume road. Imagine the safeness of trying to Parallel Park during peak hour traffic.

The Supplemental EIS also discusses cut through traffic. Eliminating and compensating local business for the loss of close parking and reconfiguration of the lanes to allow some more volume is one solution. The City and MN DOT could also create one way pairs for the highway through town to allow for more traffic flow. The reality is the existence and texture of Stillwater is formed by its relationship to the river and the existing crossing. Operational improvements without the following pricing would likely significantly increase the traffic leaving the Stillwater residents with equally bad congestion in their town.

Changing the bridge to a non-motorized facility essentially changes the price for an auto to cross from zero to infinity. What if it was only changed to $5 and that it was a variable toll to address some of the Stillwater residents’ concerns about congestion. The congestion, I would guess is on Friday evenings in the summer when city dwellers rush to their weekend retreats. Such a toll reduces the traffic and also generates revenue to repair and maintain the existing bridge. The existing bridge is considered historic and historic for carrying cars across the St. Croix River. It seems the 4F work on the bridge did not respect the fact that the bridge’s vehicular history and the scenic views that were obtained by all the folks driving through Stillwater and across the bridge. To be scenic you need people to see the beauty. I would argue that removing motorized vehicles from the existing bridge is a direct and adverse impact to the scenic river and is not allowed unless otherwise approved by the US congress.

The value of the existing bridge has never been greater and capturing some of that value through tolls provides the best revenue stream to maintain the historic bridge and address its current deficiencies and pay for operational improvements on the approach roadways. I am an equal opportunity toller and encourage appropriate charges to the gondolas and sightseeing boats that pass under the lift bridge. Freight barges no longer utilize this stretch of the St. Croix. The bridge lift schedule should not be fixed but based on price. Rush hour lifts for tourists to pass under the bridge should be evaluated based on prices the “overs” versus “unders” are willing to pay. Are the pedestrians and bikers willing to pay to maintain the bridge? If motorized vehicles are prohibited, what are the implications to the very limited use the bridge will have during the November to March time frame? The reduced value of the bridge by eliminating the cars is the biggest threat to maintaining the historic structure. The current bridge also has the huge value of simply existing and not subject to the regulatory capture of the regulating agency for new structures.

The Stillwater lift bridge is a man-made bridge, historic and integral part of the scenic
river. Review of Google images of the Lower St. Croix River, show the
lift bridge may be the most popular image per linear foot of river it occupies. Why would it not be possible to build (if needed) a bridge that would age gracefully and be equally accepted into the eco-system. The current unconditional discrimination against massiveness and man-made form denies the man- made massive existing historic lift bridge, the center piece of the scenic lower St. Croix River.

Jason Scheppers
(Crossed the beautiful scenic lower St. Croix River twice in the past year (in a car))”

Weak Bridge


Suppose there were signs on each bridge saying whether or not it was “structurally deficient”.
Would this encourage people to take investment seriously?
Or would people route around structurally deficient bridges and get into more crashes, with a net increase in fatalities, given that the likelihood of dying on a bridge collapse is quite small compared to other causes of death.

Fallen slab is concrete proof Montreal’s crumbling

Fallen Slab, Montreal Quebec
Fallen Slab, Montreal Quebec

Toronto Star:Fallen slab is concrete proof Montreal’s crumbling

MONTREAL—A huge concrete slab fell Sunday on a major expressway that runs under downtown Montreal, the latest in a series of incidents that point to the city’s crumbling infrastructure.
No one was injured in the collapse in the Ville-Marie tunnel, but the incident could have had disastrous consequences if it had occurred on Monday at the same time, during rush hour traffic, police said.
“Our officers arrived at the scene and we verified and made sure that no one was stuck underneath the rubble,” Daniel Thibaudeau, spokesman for Quebec provincial police, told reporters Sunday.
About 100,000 vehicles use the expressway during an average weekday, according to Transport Quebec.

It is somehow reassuring when infrastructure in other countries is falling apart too.

County wheelage tax might pay for a new bridge in Minneapolis | StarTribune.com

Strib writes: County wheelage tax might pay for a new bridge in Minneapolis :

Hennepin County will consider reviving a wheelage tax it hasn’t used since 1975 to pay down debt on the Lowry Avenue Bridge.
Hennepin County could bring back a tax it hasn’t used in 36 years to help pay its share of the Lowry Avenue Bridge, now under construction in northeast Minneapolis.
It’s called a wheelage tax, and it’s collected from vehicle owners in five of the seven metro-area counties able to impose it under state law. Only Ramsey and Hennepin don’t have it.
That could change Tuesday, when the Hennepin County Board will take up a resolution by Commissioner Peter McLaughlin to charge $5 per vehicle (except motorcycles and some trailers) starting next year.
The $4 million that the wheelage tax would generate annually would be used to pay down most of the county’s $51.7 million debt on the bridge, slated to be finished next summer.
McLaughlin said that the new tax revenue would replace the property taxes now used to finance the bridge. If the board approves the wheelage tax, he said, the property tax levy would be reduced by a corresponding amount.
“Historically, we haven’t used debt backed by property taxes to pay for roads. We’ve used gas taxes and user fees like that,” he said. “Property taxes are not how we ought to be subsidizing roads.””

OK, user fees are best of all, wheelage taxes are better than sales or income taxes, and property taxes are not as good as Transportation Utility Fees or Land Value Taxes, but if accessibility increases property value, property taxes are not an unreasonable place to start for paying for roads.
The article does mention that “Minus administrative payments, the tax would generate a little more than $4 million.”, I assume it is collected by the state with annual vehicle registration. I would hate for there to be a new tax collection infrastructure for this.
Also, why aren’t bicycles assessed (at half rate since they have only two wheels)? (Yes that was rhetorical).

China’s record-breaking Jiaozhou bridge ‘is safe’

Jiaozhou Bridge, China
Jiaozhou Bridge, China

A reader submits this: BBC News says China’s record-breaking Jiaozhou bridge ‘is safe’.
Here the ‘scare quotes’ are entirely appropriate.

The chief engineer of the world’s longest sea-bridge, in China, has denied claims that its construction was rushed to allow it to open on schedule.
Shao Xinpeng told state media that the Jiaozhou bridge, opened last Thursday, was safe and ready for traffic.
Chinese media reported finding incomplete crash-barriers, missing lighting and loose nuts on guard-rails.
Reports blamed workers’ haste to finish the bridge in time for the Communist Party’s 90th anniversary.
In a report earlier this week, a journalist from the state-run CCTV news channel unscrewed pieces of the guard-rails and showed that the lighting system was not working properly.
Construction workers told CCTV that it would take two months before finishing all of the projects related to the bridge.
But Mr Shao said the problems highlighted in the reports were not major.
“The status of secondary features does not affect the main project or the opening of the bridge,” he told the state-run Xinhua news agency.
He added that the lighting system was only aesthetic.
The structure spans 42.4km (26.3 miles), connecting the eastern coastal city of Qingdao to the suburb of Huangdao, in Jiaozhou Bay.
State media say the bridge passed construction tests last Monday and it opened to traffic on Thursday.
It is 4km longer than the previous world record-holder, the Lake Pontchartrain Causeway in the US state of Louisiana.

There are also disputes about this being the longest bridge over water in the world, since it is curved, while the Lake Pontchartrain Causeway is straight, and so goes over a longer body of water. Wikipedia writes “There is no standard way to measure the total length of a bridge. Some bridges are measured from the beginning of the entrance ramp to the end of the exit ramp. Some are measured from shoreline to shoreline. Yet others are the length of the total construction involved in building the bridge. Since there is no standard, no ranking of a bridge should be assumed because of its position in the list. ” Also note many bridges are longer (and in China) but not over water.

Another potential shutdown casualty: Stillwater Lift Bridge

Updated June 27, 2011 (at bottom)

The Pioneer Press reportsAnother potential shutdown casualty: Stillwater Lift Bridge

The Stillwater Lift Bridge would likely close during a state government shutdown, city officials have been told.
The employee who operates the bridge is not expected to be classified as critical, and the bridge would be left in the up position to allow river navigation.
It would close to traffic starting June 30.
About 18,000 cars use the lift bridge on an average day, said Stillwater Mayor Ken Harycki, with 25,000 or so on peak summer weekends. Traffic would be routed over the Interstate 94 bridge.

Some points:
(1) This is of course stupid for a variety of reasons, and would not occur but for the needless politicization of transportation. No other public utility would find itself shut down because of the state budget problem. Imagine they turned off electricity, or water, or even transit.
(2) This will make an excellent experiment on the importance (or lack) of this bridge. If only someone were doing before, during, and after studies. Of course with the shutdown, no one would get paid to do a “during” study.
(3) The evidence that the facility is not considered “critical” is telling about its importance.
Update June 27, 2011 with Pioneer Press article

The Stillwater Lift Bridge could remain open even if a state government shutdown occurs Friday, Minnesota Department of Transportation officials said this morning.
MnDOT is including operation of the lift bridge as a core critical function in its contingency planning efforts, pending a final court ruling.
After consulting with public safety officials and healthcare providers over the past several days, MnDOT officials have determined that the lift bridge is a “core service critical to maintaining life and health safety,” said Kevin Gutknecht, a spokesman for MnDOT.
“We looked pretty hard at that, and we understand the issues so it seemed to like a good thing to do,” Gutknecht said. The lift bridge will remain open primarily to allow ambulance traffic to continue between Minnesota and Wisconsin.
Republican legislators have scheduled a 1 p.m. news conference near the bridge to urge Gov. Mark Dayton to continue delivery of critical services such as the bridge in the event of a government shutdown.
The news conference was planned before MnDOT made its announcement.

I guess MnDOT reads the blog and concluded the optics of a shutdown would look bad for this and for a potential replacement bridge.